Fusible protective devices



NGV 21, 1951 I K. w. swAlN ET AL 3,010,000

EUSIELE PROTECTIVE DEVICES Filed. Nov. 14, 1960 111111 'lll/f l C0804? H2O acrylonilrilebutadiene- (PCSfef 0f PO'S) styrene copolymer Kenneth W- Swain, Hurry H. Halles United States Patent O mut Company, Newburyport, Mass.

Filed Nov. 14, 1960, Ser. No. 68,828 Claims. (Cl. 200-131) This invention relates to low-voltage electric fuses and more particularly to so-called one-time fuses.

It is a general object of this invention to provide onetime fuses which have a relatively high interrupting capacity, and whose casings have a higher dimensional stability and are less expensive to manufacture than those of prior art fuses having an equal interrupting capacity rating.

It is possible to classify low-voltage fuses depending upon the nature of the material used for making the casing or fuse tube thereof. From this point of view the following types of fuses may be distinguished:

(l) Fuses having a fuse tube or casing of paper or substantially equivalent organic material;

(2) Fuses having a fuse tube or casing of vulcanized fiber or substantially equivalent organic material;

(3) Fuses having a fuse tube or casing of a syntheticresin-glass-cloth laminate, and

(4) Fuses having a fuse tube or casing of a ceramic material.

The performance characteristics of the material of the fuse tube or casing have important effects upon the design and the performance of the fuse as a whole. Fuses having fuse tubes or casings of a suitable synthetic-resinglass-cloth laminate can withstand very high internal pressure and heat shock, and also withstand very well for a short time the heat of an arc, but must be designed in such a way that the temperature of the fuse tube or casing stays nonnally at a relatively low level. Fuses having fuse tubes or casings of a ceramic material tend to have a more limited resistance against internal pressures and heat shock, but their fuse tubes or casings are allowed to assume relatively high temperatures while performing their normal current-carrying duty.

This invention refers to fuses having characteristics generally similar to those of fuses including fuse tubes or casings of vulcanized ber, but being significantly improved in several respects, particularly in regard to dimensional stability under adverse operating conditions and in regard to cost of manufacture. Fuses according to the present invention are not designed nor intended to perform in exactly the same fashion as fuses having much more expensive casings of synthetic-resin-glasscloth laminates or ceramic materials. They are designed and intended to have all the desirable performance characteristics of fuses having fuse tubes or casings of vulcanized fiber, but to. be free of many of the limitations of fuses having fuse tubes or casings of vulcanized fibers, and to be much less expensive to manufacture than comp-arable fuses having fuse tubes or casings of vulcanized fiber.

To impart a relatively high interrupting capacity to fuses calls for fusible elements made o-f metals combining relatively high conductivity and relatively low fusing energy. Metals such as silver or copper having a relatively high fusing point comply best with these requirements. Fuses manufactured at the same general price level as `fuses having casings of vulcanized ber, of fuses having casings of vulcanized ber, call for fusible elements of copper rather than fusible elements of silver. The arc products resulting from vaporization of silver and copper, particularly the vapors of copper, are of a very destructive nature and require certain performance characteristics for fuse casings inherent in vulcanized fiber but lacking in other organic materials.

It is, therefore, another object of this invention to provide one-time low voltage fuses having fusible elements of a metal whose conductivity and melting point are relatively high, eg. copper, which fuses have casings Whose ydimensional stability is higher than that of vulcanized fiber, and which fuses are capable of withstanding without deterioration the thermal effects of electric arcs.

Most plastic substances have been investigated with a view of determining their applicability as fuse tubes or casings in low voltage fuses. The outcome of these investigations which have been carried out by a relatively large number of independent investigators have been largely negative, i.e, most plastics have been found unsatisfactory if not laminated with glass liber cloth. While some laminates have outstanding performance characteristics as fuse tubes or fuse casings, the cost of the laminating process is relatively high, and therefore such fuse tubes or fuse casings can only be applied for manufacturing fuses that must meet the highest performance standards and can be priced accordingly.

It is, therefore, another object of this invention to provide low voltage fuses having a relatively high interrupting capacity whose casing is made of a non-laminated plastic such as, for instance, an extruded plastic tube, which has desirable mechanical properties and is not subject to damage by arcing incident to blowing of the fuse.

During our search for suitable synthetic or plastic casing materials for low voltage fuses we investigated among many other such materials acrylonitrile-butadiene-styrene copolymers. Because of their favorable mechanical properties and their relatively low price these materials seemed to be attractive for the purpose in hand. This family of synthetic rubbers disintegrates, however, at temperatures well below arcing temperatures and for this reason there was initially little hope that it could be used for manufacturing casings for llow-voltage fuses. We made experimental fuses having copper links, casings of acrylontrile-butadiene-styrene copolymers, and an arc-quenching quartz sand ller. These fuses failed completely when tested under relatively severe interrupting conditions. The quartz sand was then replaced by a woven glass liber sleeve as described in United State Patent 2,833,890 to P. C. Jacobs, Jr., Fillerless One Time Fuse, May 6, 1958. These fuses failed likewise completely when tested under relatively severe interrupting conditions. We then made test fuses having fusible elements of a metal having a relatively high conductivity and a relatively high fusing point, i.e. copper, having casings of an acrylonitrile-butadiene-styrene copolymer and having a non-fulgurite-forming pulverulent ller of an inorganic material evolving gas under the heat of an are separating the casing from the fusible element. These fuses performed satisfactorily on heat runs and under various interrupting condi-tions, including very severe interrupting conditions. They fully conformed to all the test conditions imposed by the Underwriter Laboratories.

It ought to be added to the foregoing that the success o-f our aforementioned tests depended on several factors including the substantial absence of carbon in the acryl'onitrile-butadiene-styrene copolymer used for making the casings or fuse tubes. Commercially availabe acrylonitrile-butadiene-styrene copolymers generally include a substantial amount of carbon readily ascertainable by the dark color of the product and resulting in tracking and concomitant failure if applied for making fuse tubes or fuse casings. A substantially carbon-free acrylonitrilebutadiene-styrene copolymer is commercially available from Plastics Naugatuck Chemical Division of `the United States Rubber Company, Naugatuck, Connecticut, and marketed as Kralastic HTHT.

The foregoing and other general and special objects of the invention and advantages thereof will more clearly appear from the ensuing panticular description of the invention as illustrated in the accompanying drawing wherein:

FIG. 1 is a longitudinal section of a fuse embodying this invention taken along 1-1 of FIG. 2, and

FIG. 2 is a longitudinal section of the same fuse as shown in FIG. 1 taken along 2 2 of FIG. l.

The fuse embodying this invention comprises a tubular casing 1 of a thermoplastic substantially carbon-free acrylonitrile-butadiene-styrene copolymer, e.g. the aforementioned Kralastic HTI-IT whose color, when substantially carbon-free, is an oit-white. Casing 1 consists of a length of homogeneous extruded pipe, i.e. it does not comprise any glass-cloth layer. 'Ihe fusible element in form of multiperforated copper ribbon 2 is inserted into casing 1. The axially outer ends of casing 1 fonm rims 1a, 1a and ribbon 2 has points 2a engaging angularly displaced points of rims 1a. In the embodiment of the invention shown, the points at which ribbon 2 engages rims 1a, 1a are angularly displaced 180 degrees. Ribbon 2 has axially outer ends 2b bent around the points of rims 1a, la engaged by ribbon 2 to the outer surface of casing of fuse tube 1. A pair of metal caps 3 for closing casing 1 and for connecting ribbon link 2 into an electric circuit is mounted on the axially ou-ter ends of casing 1. Caps 3 and ribbon 2 are conductively connected by internal solder joints (not shown), more fully described in the copending patent applications of Frederick I. Kozacka Ser. No. 627,624, filed December ll, 1956, for Manufacturing Cartridge Fuses and Ser. No. 72,945, iiled Dec. 1, 1960, for Electric Fuses Having Improved Cap-Link Connection. Ribbon 2 is provided with a plurality of perforations or points of reduced cross-section 2c. It supports an overlay 2d of a low fusing point metal, e.g. tin, for interrupting the current path through ribbon 2 upon fusion of overlay 2d. Upon fusion of overlay 2d the overlay metal and the base metal, e.g. copper, react metallurgically, and as a result of such reaction the current path through ribbon 2 is severed, i.e. a `break is formed at the point where overlay 2c is located. Low fusing point metal overlays forming a severing means for high fusing point fusible elements are well known n the fuse art and are generally used as a means for achieving time-delay as more fully set forth, for instance, in United States Patent 2,703,352 to Frederick I. Kozacka, May 1, 1955, Fuse and Link of the Time Lag Type. In fuses embodying our invention the overlay 2d has an important secondary function, i.e. a function other than that of producing time delay. Since the tin overlay 2d is arranged at the hottest point of the fuse, the hottest point of the fuse will never exceed the fusing point of tin and this provides a sigmicant degree of thermal protection for the casing 1 of an acrylonitrile-butadiene-styrene copolymer. If the fusible ribbon element 2 Were only of copper the hottest point of the fusible element could stay for long periods of time slightly below the fusing point of this metal which is 1083 deg. C., whereas the overlay 2d precludes occurrence of operating temperatures as high as 232 deg. C. which is the fusing point of tin. A similar thermal casing protection can be achieved without resorting to overlays for severing the fusible element 2 by associating the fusible element 2 with 4low fusing point fusible means other than overlays for interrupting the current path through the fusible element upon fusion of said fusible means. Appropriate low fusing point fusible means other than overlays have been more fully disclosed in United States Patent 2,321,711 to Elmer H. Taylor, lune 15, 1943, for Fusible Electric Protective Device. The casing or fuse tube contains a non-fulgurite-forming pulverulent liller 4 of an inorganic material evolving gas under the heat of an arc. Various such fillers may be used as, for instance, chalk. However, the best results were obtained with calcium sulfate in the form of CaSOil/zHZO. In that form, even if exposed to the heat of an electric arc, the calcium sulfate does not heat significantly over the boiling point of water, 212 deg. F., until the water combined with it is driven olf. The heat distortion point of acrylonitriie-butadiene-styrene copolymers at the pressures occurring in fuses is above the boiling point of water and the ignition temperature of acrylonitrile-butadiene-styrene copolymers is in the order of 600 deg. F. It is apparent from the experiments which We have conducted that an arc-quenching filler of calcium sulfate provides sucient thermal protection to a casing of acrylonitrile-butadienestyrene copolymers to prevent thermal damage to the latter on account of Iarcing inside of the casing, whereas an arc-quenching iiller of quartz sand, though having a considerably higher energy absorbing capacity than a ller of calcium sulfate, exposes the casing for a suiciently long time to temperatures sufficiently high to cause serious damage to it and thermal destruction thereof.

A filler such as calcium sulfate having a relatively limited energy absorbing capacity, or heat absorbing capacity, tends to produce a smaller arc voltage per break than a ller having a relatively high energy absorbing capacity, or heat absorbing capacity. This tendency must be compensated by providing a relatively large number of necks or points of reduced cross-section 2c of which those nearest to the axially outer ends of the casing 1 have a relatively limited spacing from the inner surface of the latter.

As mentioned above fuses embodying the present invention have successfully passed the tests imposed upon fuses by the Underwriter Laboratories. Fuses embodying the present invention are capable of successfully interrupting relatively low currents as well as relatively high currents. We are not entirely familiar with the details of all the phenomena underlying the operation of fuses embodying our invention, and the reasons for their successful operation. In addition to the foregoing, the following is oifered as an attempt of a partial explanation. If on blowing of a fuse the inner surfaces of a fuse casing of ber are hit by hot products of arcing, such as copper vapors, the fiber releases protective gases, and the deterioration of the ber is very small if the duration of the action of the hot arc products is relatively short. Acrylon ninile-butadiene-styrene copolymers have no comparable self-protecting action. Comparing a quartz filler and la filler of calcium sulfate of substantially equal grain sizes, the latter is a much better heat i11- sulator and less permeable to hot arc gases. The heat momentarily trapped by the calcium sulfate filler in the region immediately adjacent the arc path is thereafter largely dissipated in a direction longitudinally of the fuse tube, and is thus kept away from acting adversely upon the fuse tube. The magnitude of the axial heat flow compared to that of radial heat flow explains also, at least in part, the amazing fact that the thermoplastic casing material can be kept below its softening point during the normal current-carrying duty of the fuse.

It will be understood that we have illustrated and described herein a preferred embodiment of our invention and that various alterations may be made in the details thereof without departing from the invention as deined in the appended claims.

We claim:

1. A fusible protective device comprising a tubular casing consisting substantially of a thermoplastic substantially carbon-free acrylonitrile-butadiene-styrene copolymer; an elongated fusible element of a metal having a relatively high conductivity and a relatively high fusing point housed in said casing; fusible means having a relatively low fusing point associated with said element for interrupting the current path through said element upon fusion of said fusible means; means for closing said casing and for connecting said element into an electric circuit;

and a non-fulgurite-forming pulverulent ller of an inorganic material evolving gas under the heat of an arc separating said casing from said element.

2. A fusible protective device comprising a tubular casing consisting substantially of a thermoplastic substantially carbon-free acrylonitrile-butadiene-styrene copolymer; an elongated fusible element formed by a perforated copper ribbon housed in said casing; an overlay of a low fusing point metal on said ribbon for interrupting the current path through said ribbon upon fusion of said overlay; means for closing said casing and for connecting said ribbon into an electric circuit; and a filler 'of pulverulent calcium sulfate separating said casing from said ribbon.

3. A fusible protective device comprising a tubular casing consisting of a length of homogeneous extruded pipe of a substantially carbon-free acrylonitrile-butadienestyrene copolymer, an elongated fusible element of a metal having a relatively high conductivity and a relatively high fusing point housed in said casing; fusible means having a relatively low fusing point associated with said element for interrupting the current path through said element upon fusion of said fusible means; means for closing said casing and for connecting said element into an electric circuit; and a non-fulgurite-forming pulverulent ller of an inorganic material evolving gas under the heat of an arc separating said casing from said element.

4. A fusible protective device comprising a tubular casing consisting of a length of homogeneous extruded pipe of a substantially carbon-free acrylonitrile-butadienestyrene copolymer; an elongated fusible element formed by a perforated copper ribbon housed in said casing; an overlay of a low fusing point metal on said ribbon for interrupting the current path through said ribbon upon fusion of said overlay; means for closing said casing and for connecting said ribbon intoy an electric circuit; and a filler of pulverulent calcium sulfate separating said casing from said ribbon.

S. A fusible protective device comprising a tubular casing consisting of a length of homogeneous extruded pipe of a substantially carbon-free acrylonitrile-butadienestyrene copolymer forming a pair of rims at the axially outer ends thereof; a multi-perforated copper ribbon inserted into said casing, said ribbon having points engaging angularly displaced points of said pair of rims and having axially outer ends bent around said points of said pair of rims to the outer surface of said casing; an overlay of a low Ifusing point metal on said ribbon for interrupting the current path through said ribbon upon fusion of said overlay; a pair of metal caps for closing said casing and for connecting said ribbon into an electric circuit mounted on said axially outer ends of said casing; and a ller of pulverulent calcium sulfate separating said casing from said ribbon.

References Cited in the lile of this patent UNITED STATES PATENTS Murray May 4, 1909 Eustce et al Sept. 10, 1918 OTHER REFERENCES 

